There are many types of electrically powered apparatus having a remote primary control station from which energization of the apparatus is usually controlled, and a secondary control station which is remote from the primary station, and usually close to the apparatus, from which energization of the apparatus may also be controlled. Typical of such secondary control stations are jog control stations which are placed close to the electrically powered apparatus so that a person servicing the apparatus may operate it from a conveniently located control station.
Typical of such apparatus are silo unloaders, and the present invention is described as applied to a silo unloader, although obviously it has broad applicability to many types of electrically powered apparatus.
A silo unloader is suspended from a tripod in the top of a silo, and may be lowered onto the top of the silage by a winch when it is necessary to remove some silage from the silo for animal feeding. The most popular types of silo unloaders have a sweep arm which extends from an area near the vertical axis of the silo to the silo wall and which is equipped with a cutting and conveying auger. The silo unloader is rotated slowly about the axis of the silo to move the sweep arm over the surface of the silage so as to cut silage from the top of the mass stored in the silo and convey it to the central area where it is picked up by an impeller that flings it through a chute connected to an open doorway in the silo wall.
The silo unloader is powered by a large electric motor, and energization of the motor is controlled from a primary control station which is commonly mounted on the outside of the silo wall near the ground, although it may be in an adjacent barn or shed. A typical silo unloader control station has a spring loaded normally open start switch and a spring loaded stop switch. A holding circuit permits extended operation of the silo unloader to be initiated by momentarily depressing the start switch to close the motor energizing circuit, and energization may be terminated by pushing the stop switch. In addition, it is usual for a silo unloader control station to have a timer actuated switch in the holding circuit so that an operator may set the timer to run the silo unloader for any desired period of time.
When it is necessary to service a silo unloader, the operator climbs a ladder on the outside of the silo, enters the silo through the open silage discharge door and stands on the silage while he works on the unloader. In common with most electrically powered apparatus which has a remote primary control station, there is a jog control station mounted on the silo unloader close to the motor, so the operator who is servicing the unloader may energize the motor to test unloader operation. Such a jog control station commonly has a spring loaded push button start switch with no holding circuit, so the motor is energized only as long as the operator manually depresses the start button.
Usually there is no way to disable a remote primary control station so as to eliminate the possibility that somebody will start the apparatus in ignorance of the fact that there is a person in the silo working on it. This makes it necessary for the operator to hang a warning sign on the primary control station before he enters the silo to service the unloader, but this is an easy thing to forget and requires that a warning sign be kept conveniently close to the primary control station.
Insofar as applicants are aware, there has heretofore been no simple and reliable means for disabling a remote primary control station when an operator is about to work on apparatus which may be started from the primary control station. The problem is particularly acute in the case of silo unloaders and other like equipment where the apparatus is not visible from the primary control station.